2016-36-0126
Stiffness and vibration damping capacity of high strength cast irons
W. L. Guesser, L. P. R. Martins
TUPY/UDESC
Copyright © 2016 SAE International
Abstract
The trend to lightweight design of automotive engines has led to the
development of new cast iron grades for cylinder blocks, with very
high fatigue properties, resulting in engines in some cases even
lighter than engines with cylinder blocks of aluminum. On the other
hand, the selection of cast irons grades with high values of
mechanical strength and high elastic modulus, for projects of thinwall engine blocks, may result in decrease in vibration damping
capacity, even still far superior to aluminum cylinder blocks. This
paper deals with damping capacity and elastic modulus of high
strength cast irons, considering how the microstructure affects these
properties and how to optimize them.
Introduction
The automotive industry has made continuous improvements aimed
at reducing the consumption of fuel, with results as shown in Figure
1, for passenger cars and light trucks in USA. The fuel consumption
was reduced by about 50 % for passenger cars between 1980 and
2014. For light trucks the reduction was smaller, about 30 %, but it is
still worthy of emphasis.
engine operation (3). Thus, oil consumption and emissions are
affected by elastic modulus of the material used for the construction
of engine blocks. Lower bore distortion allows for reduced ring
tension and thus reduced friction losses (4). On the other hand,
increasing the elastic modulus normally results in decreasing
damping capacity, another very important parameter for automotive
parts. So, in the present study both properties, elastic modulus and
damping capacity are measured, in cast irons with different
microstructures.
Experimental procedures
The tests were conducted with two ductile irons (grades 450 and
700), two compacted graphite irons (grades 450 and 500) and seven
gray irons (grades 250 and 300), as shown in Table 1. Samples of
ductile iron and CGI were casted into 25 mm Y block and gray irons
were molded into standard bar of 30 mm diameter. For measuring the
physical properties, it was used resonant frequency test. Hence, CGI
resonance bars were machined with square section of 20 mm, 120
mm length, whereas gray and ductile iron specimens were machined
in bars with 25 mm diameter, 120 mm length.
Different combinations of alloying elements and inoculation were
used for the gray iron samples. For the CGI, in order to evaluate the
effect of the cooling rate, the samples were machined from two
positions of the Y block, the usual bottom position (5 mm from the
bottom as cast surface) and at an upper position (50 mm from the
bottom).
Table 1 – Cast irons tested on the present work.
Cast Iron
Grade
Alloying elements
Ductile Iron
DI 450
-
DI 800
0,6Cu
CGI 450
1,0Cu - 0,09Sn
CGI 500
1,0Cu - 0,3Mo
GI 250
0,9Cu-0,7Sn-0,27Cr
GI 300-1
0,7Cu-0,06Sn-0,24Mo
GI 300-2
1Cu–0,09Sn-0,29Cr
GI 300-3
0,6Cu-0,06Sn-0,24Cr (*)
GI 300-4
0,9Cu-0,09Sn-0,3Mo (*)
GI-300-5
0,6Cu-0,06Sn-0,2Cr-1,5Ni
GI 300-6
0,7Cu-0,04Sn-0,2Cr-0,27Mo
Compacted Graphite Iron
Figure 1 – Fuel economy performance of passenger cars and light trucks in
USA. NHTSA (1).
Among the various technologies which allow this result, a decrease in
vehicle weight was an important factor. In particular, the use of
higher strength materials had significant contribution to various
vehicle components. In the engine, the use of high strength cast iron
grades, such as gray iron grade 300 (UTS min = 300 MPa) and
compacted graphite iron grade 450 (UTS min = 450 MPa), has been a
very important tool for engine designs of lower weight and higher
specific output (2).
Apart from the fatigue strength, a very important property for engine
blocks is the elastic modulus. This property measures the stiffness of
the material, which will maintain the shape of the cylinders during
Gray Iron
(*) – Special inoculation to refine graphite.
The microstructures of the cast irons evaluated on this work can be
seen on figure 2.
gray), the elastic modulus values are affected by the grade, especially
for gray iron. In this case, the change from grade 250 to 300 is
obtained by using alloying elements and particularly by reducing the
quantity and refining of graphite. This graphite size change affects
the propagation of the elastic wave in the material, thus influencing
both the elastic modulus values as the damping.
Figure 4 – Elastic modulus and damping capacity of cast irons.
Figure 2– Typical microstructures of ductile iron (grade 800), CGI (grade
450) and gray iron (grade 300), with Nital etching.
The resonance frequency test was carried out using the impulse
excitation method (5). The method, illustrated in Figure 3, uses the
software Sonelastic ® (6). This consists of an analyzer of transient
vibrations, which obtain the resonant frequency for calculating the
elastic modulus, and the respective decay rates for the evaluation of
damping capacity (7). The damping is calculated by logarithmic
decrement technique, which considers the ratio of two successive
amplitude signals (8).
Considering the difference in damping capacity and elastic modulus
between the grades GI 250 and GI 300, and between gray and
compacted graphite irons, complementary experiments were
conducted. It was investigated in detail the effect of microstructure
on the damping capacity in the grade GI 300, using 7 different gray
irons. It was also evaluated the elastic properties of two types of CGI.
Special attention on these families of cast irons (gray and CGI) is
driven by its increasing use in modern engines cylinder blocks and
heads.
The metallographic and mechanical characteristics of CGI samples
are shown in table II. It can be seen that the addition of Mo increases
the mechanical strength of CGI and that in the lower position of the
Y block the mechanical properties are higher compared to the top
position. This is a result of cooling speed differences, which also
translate in the metallographic characteristics of pearlite and graphite.
Mo also resulted in refining of pearlite.
Figure 3 - Scheme of the experimental apparatus used in the
resonance frequency test. [7], [8]
Results and Discussion
Figure 4 shows results of elastic modulus and damping capacity,
evaluated by vibration tests. It is confirmed that ductile irons have the
highest modulus and the lowest values of damping capacity, while
gray irons show high levels of damping capacity and the lowest
elastic modulus values. Compacted graphite irons are among the
ductile and gray irons. In each cast iron family (nodular, compacted,
Page 2 of 6
In figure 5 we can observe the results of elastic modulus and damping
capacity, for the CGI samples. The elastic modulus increases with
increasing cooling rate (bottom position) and with addition of Mo.
This is the result of increased nodularity, refining of compacted
graphite (eutectic cells count and graphite particles count) and
refining the pearlite. The damping capacity also increases with
increasing cooling rate (bottom position), probably due to the refine
of the graphite particles and perlite; the effect of Mo it is not clear in
this case. As a general observation, for CGI both elastic modulus and
damping capacity increase when the microstructure is refined
(pearlite and graphite).
Table II- Tensile test results and metallographic analysis of CGI samples. 100
% pearlite. Graphite shape III and VI.
Sample
CGI
bottom
CGI
upper
CGI Mo bottom
CGI Mo
- upper
UTS (MPa)
458
450
530
514
YS (MPa)
361
353
412
413
Nodulization (%)
6
5
7
6
Eutectic cell count/cm2
677
575
531
518
Graphite particules
count/mm2
508
490
502
489
Pearlite interlamelar spacing
(µm)
0,32
0,35
0,25
0,29
Figure 5 - Results of elastic modulus and damping capacity of CGI samples,
in bottom and upper position of the Y block.
Page 3 of 6
Figure 6 shows, for gray irons grades GI 250 and GI 300, the
relationship between certain parameters of the microstructure, in
particular some aspects of graphite. The number of eutectic cells
reflects the conditions of nucleation during solidification, while the
number of graphite particles (measured on a metallographic section)
is the result primarily of graphite growth conditions. Both parameters
relates to the undercooling during solidification, and therefore present
a high correlation with each other. The amount of graphite, for cast
iron of the same carbon equivalent content, is also influenced by the
solidification undercooling, decreasing with the increase of this
parameter. In this case, as the geometry of the specimens was always
the same (30 mm diameter bars), the variation of the undercooling
was a result of different inoculations and alloying elements. We can
see that the amount of graphite decreases with increasing number of
graphite particles and eutectic cells.
Figures 7 and 8 illustrate the effects of graphite on tensile strength
and Elastic Modulus in gray irons. It can be seen that both Elastic
Modulus and UTS increase with increasing number of eutectic cells
and with the number of graphite particles and decrease with
increasing graphite amount. It thus appears that the microstructural
factors related to graphite affect likewise the material stiffness
(Elastic Modulus) and its fracture strength (UTS). In this case, during
the process of fracture of the material, increasing the number of
eutectic cells and refining the graphite particles imposes difficulty in
the progress of cracks, while increasing the amount of graphite
facilitate crack growth.
Figure 6 – Relationship between graphite microscopic characteristics in gray
irons. Bars with 30 mm diameter, with CE = 4,0-4,1. The amount of graphite
decreases with increasing number of graphite particles and eutectic cells.
Page 4 of 6
Figure 7 – Relationship between graphite microscopic characteristics and
tensile strength in gray irons. Bars with 30 mm diameter. UTS increases with
higher eutectic cell number, refined graphite and lower amount of graphite.
Figure 9 also shows the inverse relationship between elastic modulus
and damping capacity for the samples of gray irons under
examination, so that high values of elastic modulus are associated
with low values of damping capacity. The microstructure
characteristics that most influence the damping capacity are the
number of eutectic cells and the amount of graphite (Figure 10).
There was no observed relationship between the refining of graphite
and damping capacity. Thus, to maximize the damping capacity
should be selected gray irons with large amounts of graphite and
large eutectic cells.
Figure 9 – Relationship between elastic modulus and damping capacity.
Inverse relationship.
Figure 8 – Relationship between graphite microscopic characteristics and
elastic modulus in gray irons. Bars with 30 mm diameter. Elastic modulus
increases with higher eutectic cell number, refined graphite and lower amount
of graphite, the same trend for UTS.
Page 5 of 6
Diesel: aujourd’hui et demain. Lion. 2004.
3. Guesser, W. L. Propriedades Mecânicas dos Ferros Fundidos.Ed
Edgard Blücher, 2009.
4. Dawson, S. & Indra, F. Compacted graphite iron – a new material
for highly stressed cylinder blocks and cylinder heads. 28th
International Vienna Motor Symposium, 2007.
5. ASTM E1876, “Standard Test Method for Dynamic Young’s
Modulus, Shear Modulus, and Poisson’s Ratio by Impulse
Excitation of Vibration”. Feb 2001.
6. E. F. ATCP, “ATCP - Engenharia Física,” 2011. [Online].
Available: www.atcp.com.br.
7. Cosolino, L C & Pereira, A H A. Módulos elásticos: visão geral e
métodos de caracterização. 2012. www.atcp.com.br.
8. G. Roebben, B. Bollen, A. Brebels, J. V. Humbeeck e O. V. d.
Biest, “Impulse excitation apparatus to measure resonant
frequencies, elastic,” American Institute of Physics, pp. 45114515, Dezembro 1997.
Contact Information
W. L. Guesser. [email protected]
L. P. R. Martins. [email protected]
Acknowledgments
Figure 10 – Relationship between graphite microscopic characteristics and
damping capacity in gray irons. Bars with 30 mm diameter. Damping capacity
increases with lower eutectic cell number and higher amount of graphite, the
opposite trend for elastic modulus and UTS.
Conclusions
It was found that the elastic modulus of cast irons correlates with the
strength of these materials, and is dependent on graphite shape,
presenting high values for ductile irons, low values for gray irons and
intermediate values for compacted graphite iron. Inverse relationship
is recorded for the damping capacity, with high values for gray irons.
This property can still be maximized for gray irons with the design of
the microstructure, selecting gray irons with high amounts of graphite
and low number of eutectic cells. These factors are contrary to the
search for the increased strength and stiffness of gray irons, so the
selected microstructure must be a compromise between strength,
stiffness and damping capacity.
References
1. NHTSA. CAFE Summary of Fuel Economy Performance. 15
Dec 2014. http://www.nhtsa.gov/fuel-economy
2. Guesser, W L; Duran, P V; Krause, W. Compacted graphite
iron for diesel engine cylinder blocks. SIA - Congrès Le
Page 6 of 6
This paper is part of the master thesis of L. P. R. Martins/ UDESC,
Brazil, conducted in the partnership with Tupy and UDESC.
Definitions/Abbreviations
SGI
Spheroidal graphite iron
CGI
Compacted graphite iron
GI
Gray iron
USA
United State of America
UTS
Ultimate tensile strength
Cu
Copper
Sn
Tin
Mo
Molybdenum
Cr
Chrome
YS
Yield strength
E
Modulus of elasticity
G
Shear modulus
µ
Poisson coefficient
UDESC
University of State of Santa
Catarina